According to director Goichi Suda, Romeo's design was based on the main character of a cancelled project called "Zombie Rider", whose main character would have been a human-zombie hybrid. The project never came to fruition, but Suda decided to use the design for Romeo, who is similarly half-dead. These ideas were tweaked in the process, such as updating his backstory so that his grandfather Ben used the Dead Gear to bring him back to life.

Within the game's data is an unused placeholder cover image for a custom song, consisting solely of a stock image of a picture frame and comedic text congratulating the player for importing their own music into the game. The image was likely used to test the PC version's custom beatmap feature, which, incidentally, was not officially announced until nine days after the placeholder's discovery.

Originally, Marge's megaphone had a limit on how many people she could recruit into her mob. In the final version, however, there is no limit, and she can recruit as many as she wants. In the game engine, her original upgrade would increase the number of mob members she could gather, rather than boosting her blowback strength as it does in the final game.

Mewgenics includes the ability to obtain cats with real-life congenital abnormalities, playing off of the stigmatization of them in the actual eugenics movement. According to co-creator Edmund McMillen, he and co-director Tyler Glaiel received many pitches and ideas during development from fans living with these conditions on how to represent them in a positive light in the game. McMillen (who himself has dyslexia) wanted to include them to show that they can carry in-game advantages in spite of their concurrent drawbacks (i.e. cats born with dwarfism have exceptionally high luck), tying these into the game's greater theme of "making it work [with] what you're given". In line with the fan participation, the in-game portrait for autism is modeled off the TBH Creature (also known as the Autism Creature), an MS Paint drawing by Twitter user @acmeiku that went viral on Tumblr and was jokingly referred to by some as an unofficial mascot or spokesperson for the disorder.

The original mechanic for Bart's slingshot was slightly different. Instead of his ammo being tied to his power meter, Bart's default ammo would have been unlimited. Players could also collect limited special ammo throughout levels, which would increase the slingshot's range and damage. Although this system was not used in the New-Gen version, the special ammo mechanic remained in the Old-Gen and DS versions.

In the 2025 Nintendo Switch version of Super Mario Galaxy, at the start of the mission "Tarantox's Tangled Web", Captain Toad's line from the original game was changed from mentioning only a single Toad to multiple Toads, since none of the Toad Brigade went alone to go looking for the Power Star in the said mission.

The title and cover art for "ball is infinite" reference a notorious Easter egg from Sonic CD, where toggling the sound test to a specific combination produces a screen depicting multiple copies of Sonic the Hedgehog with humanlike faces and text atop translating to "Infinite fun. Sega Enterprises. –Mazin Picture". The "ball is infinite" cover art similarly depicts multiple copies of Beat mimicking Sonic's pose with Quaver's face superimposed atop hers. Also like the Sonic CD screen, the text on the "ball is infinite" cover is written in Japanese with "infinite" represented by an infinity symbol, though an English translation is provided below. A looser translation of the Sonic CD screen's text, "Fun is infinite with Sega Enterprises," forms the basis of the song title.

"SQUARE UP", a song contributed to the Breakout Edition expansion by American electronic musician Jamie Paige, prominently references her 2022 song "(:women_wrestling:)" (itself later included in the game's first paid DLC pack). Not only do both tracks utilize fistfight imagery as a metaphor for love, but "SQUARE UP" also features multiple lyrical callbacks to its predecessor. The line "But there's a bullet kept inside my zone" references the "(:women_wrestling:)" line "I keep a bullet with your name inside my zone", and the "SQUARE UP" lines "Back to back / Beat some ass" are directly sampled from the 2022 track. The callbacks to "(:women_wrestling:)" are furthered in the music video for "SQUARE UP", two scenes of which feature illustrations of Gumi wearing the same outfit from the 2022 song's cover art. Both the illustrations and the "(:women_wrestling:)" cover art are additionally done by the same artist, Bearvamps.

Originally, all 100 of the game's endings were going to be directed by co-director Kotaro Uchikoshi, with the writing being split up across the various in-house writers of Too Kyo Games. However, as the game received more content and deadlines approached, fellow co-director Kazutaka Kodaka became involved in various routes of the game. One of these routes was the "Cult of Takumi" route, which Kodaka began working on after learning that nobody else was working on a "damn sexy" route. The original plan for this route was for it to be a "sexy harem narrative", but as he wrote more, the story as seen in the final game became darker and more twisted.

According to Pokémon illustrator Yusuke Ohmura, the Ultra Beasts' designs were created with the aim of making them look as little like traditional Pokémon as possible, making use of "combinations of motifs that could never realistically exist" as well as unused Pokémon designs that Ohmura had submitted but had been rejected by primary Pokémon character designer Ken Sugimori. The reasons why these designs were rejected were used as a basis for the designs of the Ultra Beasts.

There's a first-person view mode only present in the E3 2007 build of the game that allows you to freely look around the environment; this mode can be accessed by pressing the left and right sticks together in the build. In this mode, enemies can still detect and attack you, but you won't take any damage.

Upon reaching Acquaintance level with Honey, she will have a chance to say: "My kids' friends used to call me "Stacy's mom". I've had some weird nicknames over the years, but this one stumped me... My daughter's name isn't Stacy!" This is a reference to the song "Stacy's Mom" by the American rock band Fountains of Wayne.

According to the game's director Hideki Kamiya, the Bangle of Time item and its ability to stop time was inspired by The World, a Stand from the manga series JoJo's Bizarre Adventure.

D-Hopper was originally developed and slated for release in the Summer of 1996 under the name Dragon Hopper. The game centered around Dorin, a young dragon prince who must save his family and love interest after they are all captured by a corrupt prime minister. Despite being completed, the game's release was cancelled due to the critical and commercial failure of the Virtual Boy. It would later see an official release in 2026 for the Nintendo Switch Online service alongside another unreleased Virtual Boy game, Zero Racers. It's believed the game's North American name was changed to D-Hopper because the trademark for the name "Dragon Hopper" had been claimed in 2024 by the online casino game developer Push Gaming.

Ballz 3D and Brutal: Paws of Fury are the only two known games that were intended to include support for the Edge 16, a modem add-on for the Sega Genesis developed in collaboration between PF Magic and AT&T that allowed for online multiplayer over phone lines. Though intended for release by the end of 1994, the Edge 16 never released, due to the upcoming fifth generation of consoles cutting the Genesis' lifespan short. Despite this, the North American manual for the Genesis version of Ballz 3D released with an entire section detailing how to connect to online matches using the Edge 16.

When Birdo (known in Japanese as Catherine) was first introduced in Yume Koujou: Doki-doki Panic, the game's manual gave this description:


This carried over to the manual for the game's Western reskin as Super Mario Bros. 2 (known as Super Mario USA in Japan), where it instead states that Birdo's preferred name is "Birdetta". Starting with the commercial for Super Mario USA, Birdo was depicted in Japanese media as a flamboyant and flirtatious okama (a Japanese-language derogatory slur used to refer to gay men, transgender women, male crossdressers, and/or effeminate men) in both her voice acting and mannerisms for comedic purposes. However, this depiction, as well as the Western Birdetta name, were largely omitted from English localizations of future Mario games, with some exceptions:

• Mario & Luigi: Superstar Saga, where Birdo appears as an accomplice of Popple, who feels confused and discomforted by her.
• Super Smash Bros. Brawl, where Birdo's trophy description calls Birdo "gender indeterminate", uses "it" instead of a gendered pronoun, and includes the omitted Birdetta name.
• Super Mario Party, where Birdo is referred to with she/her pronouns in the American English script, he/him pronouns in the British English script, and no gendered pronouns in the Japanese script.

Birdo's Japanese depictions drew characteristics from drag queens and transgender women in Japanese pop culture, and Birdo would retain toned down elements of these in subsequent reappearances. Birdo frequently refers to herself in Japanese media with both the first-person feminine pronoun "atashi" and the gender-neutral first-person pronoun "watashi", invoking the latter in a feminine manner. In the Satellaview game BS Super Mario USA, the three types of Birdo are voiced by Shiroi Heya nightclub performers Jun Donna, Rika, and Akemi, all of whom identified as newhalves (a Japanese term interchangeably used to refer to transgender people and male crossdressers). Birdo's gender is most explicitly addressed in Captain Rainbow, where she is depicted again as an okama and is arrested for using a women's restroom. This forms the basis for a sidequest where she can only be exonerated once the player finds a censored item in her house called the "female proof" that is strongly implied to be a vibrator. In one trailer for the game, Little Mac describes Birdo as a "young lady", before pausing and restating: "Depending on one's view point, she is a young lady."

In 2015, a representative for Nintendo told the Japanese news site Netorabo that Birdo, as well as Yoshi, were neither male or female, or even dinosaurs; Birdo was "someone who believes they are a girl [...] 'Yoshi' is 'Yoshi' and 'Catherine' is 'Catherine'".

Unlike most other characters in the game, the screen doesn't darken when Liu Kang performs his fatality. When co-designers Ed Boon and John Tobias were asked about this on Twitter, they said it was an intentional choice to highlight Liu Kang's status in the game as the good guy who doesn't actually kill his opponents.

During his retirement announcement in 2026, producer Kensuke Tanabe revealed that the heavy focus on Sylux in the plot of Metroid Prime 4: Beyond was a longtime ambition of his, stating that the idea first came to him when developing the character during the making of Metroid Prime 3: Corruption and implementing early hints at the ends of that game and Metroid Prime: Federation Force. He wanted to make Metroid Prime 4 the start of a story arc revolving around Samus' conflict with Sylux, seeing that game as his last chance to do so on account of his age. Tanabe additionally stated that he wanted a later Metroid Prime game to focus on Samus realizing that she has to confront Sylux directly on account of his nature as "a dark, resentful villain who resorts to underhanded tactics," handing off the execution of that concept to his successor, Risa Tabata.

According to series creator Ed Annunziata, he originally planned to name Ecco the Dolphin after the dolphin constellation Delphinus (a design that appears on Ecco's head in the form of scars). Sega of America's global marketing director Al Nilsen instead suggested the name Botticelli, referencing the Renaissance painter Sandro Botticelli, in an attempt to imitate how the Teenage Mutant Ninja Turtles were named after famed Italian artists. Annunziata sensed this was a bad idea and turned to Sega of America's director of product development Clyde Grossman, who suggested "Echo" based on echolocation. Annunziata then replaced the "h" in the name with a "c" to form the final name; he explained that it was a reference to the Italian word "Ecco", which can be loosely translated to English as "I see" (i.e. understanding something). It is also thought to be a reference to the works of John C. Lilly, a neuroscientist known for his controversial attempts to communicate with dolphins, who believed that he had been contacted by an alien organization called the "Earth Coincidence Control Office", or "E.C.C.O." While this connection has not been directly confirmed, Annunziata once stated in a series of tweets about Ecco the Dolphin trivia that he "never took LSD, but did read a lot from John Lilly".

Game Boy Color versions of Backyard Baseball and Backyard Football were announced in 2000, but were never released for unknown reasons.